The present invention generally relates to digital video signal reproducing apparatuses, and more particularly to a reproducing apparatus for reproducing a digital video signal from a recording medium to obtain the original analog video signal, where the digital video signal which is time-sequentially recorded on the recording medium has a signal format in which picture element data groups obtained by subjecting the analog video signal to digital pulse modulation are divided in terms of predetermined units, and a header signal at least comprising a synchronizing signal and a picture mode identification code is added to each of the divided picture element data groups.
Recently, systems which record a digital video signal obtained by subjecting video and audio signals to digital pulse modulation such as pulse code modulation (PCM) and a digital audio signal onto a rotary recording medium (hereinafter simply referred to as a disc) as variations in geometrical configuration, and reproduce the recorded signal as variations in the intensity of light reflected from the disc or variations in electrostatic capacitance, have been developed and realized. Further, recording systems have been proposed for digital audio discs, according to which a digital video signal comprising color still picture information is added to a digital audio signal and recorded together on the same track on the disc. Generally, a plurality of music programs are recorded on the same side of such a digital audio disc, and the digital video signal comprising the color still picture information is recorded in correspondence with each of the recorded music programs. When reproducing such a digital audio disc, the music programs on the disc can be reproduced by a reproducing system which is common throughout the world. However, the television systems are not common throughout the world, and there roughly exist three kinds of television systems. Accordingly, in order to enable reproduction of the video signal recorded on the disc even if the television system employed in a region or country is different from the television system of the recorded video signal, it is first necessary to convert the recorded video signal into a signal format in accordance with the television system of the reproducing apparatus used in that region or country before obtaining a reproduced picture. The information content of the above digital video signal relates to a color still picture which helps the listener's imagination when he listens to the reproduced sounds of the digital audio signal. Hence, it is desirable to reproduce the digital video signal from the disc in the signal formats which are in accordance with each of the television systems, regardless of the differences in the television systems throughout the world.
The color television systems throughout the world can roughly be divided into three systems, that is, NTSC, PAL, and SECAM systems, according to the transmission formats of the chrominance signal. In each of these color television systems, the color video signal is constituted by a luminance signal and two kinds of color difference signals. Hence, it is desirable to employ a component coding system which transmits the color video signal by independently subjecting the luminance signal and the two kinds of color difference signals to digital pulse modulation, in order to facilitate compatibility between the three systems. Moreover, it is desirable to employ the component coding system in view of the fine picture quality which may be obtained by use of a display monitor having input terminals for the three primary colors of red (R), green (G), and blue (B) which will probably be realized in the future, and especially because partial moving pictures may be recorded on the digital audio discs, and the like.
Among the digital video signals which are subjected to the component coding, the CCIR is presently studying standardization of the standards especially with respect to digital video signals which are used in television broadcasting studios. Regarding the (number of scanning lines)/(number of pictures per second) or the horizontal scanning frequency, the major systems throughout the world either belong to the system with (525 lines)/(30 pictures) or (625 lines)/(25 pictures). Hence, a component coding system is proposed to the CCIR, in which the sampling frequency of the luminance signal is set to 13.5 MHz which is a frequency equal to six times the frequency of 2.25 MHz which is a least common multiple of the horizontal scanning frequencies of the two major systems, the two kinds of color difference signals (R-Y) and (B-Y) are respectively sampled at a frequency of 6.75 MHz, and the signals are respectively quantized at a rate of 8 bits/pel. In this case, the number of sampling points of the luminance signal on one scanning line (hereinafter simply referred to as a line), is obtained by dividing the sampling frequency of 13.5 MHz by the horizontal scanning frequency of 15.625 kHz, and is equal to 864. In addition, the proposed signal format is a format which introduces no degradation in the signal even with respect to picture processing such as chromaky processing.
It is also desirable to transmit the digital video signal for home use according to the above proposed standard, however, the capacity of the picture memory element becomes large when the number of data is large, and there will be a problem in that the transmitting time of the picture becomes long. If the number of effective sampling points on one line is 720 for the luminance signal and respectively 360 for the two kinds of color difference signals (R-Y) and (B-Y) and the number of transmitting lines is 575, for example, the number of transmitted sampling points becomes (720+2.times.360).times.575=828,000. Moreover, if one sampling point is described by eight bits, the number of bits of the transmitted sampling points becomes 828,000.times.8=6,624,000. This is an information quantity which may be stored by use of 102 64k-RAMs (random access memories) having 2.sup.16 (=65,536) bits. If such an information quantity is transmitted by use of a transmission path capable of transmitting sixteen bits at the frequency of 44.1 kHz, the required transmission time is equal to 6,624,000/(44,100.times.16)=9.39 seconds. Further, if it is assumed that the memory circuit comprises two kinds of memories, that is, memory for write-in and memory for display, a total of 204 64k-RAMs become necessary. However, when transmitting the digital video signal for home use in the digital audio disc, this will make the construction of the memory circuit within the reproducing apparatus complex, and the cost of the reproducing apparatus will become high. Such complexity of the memory circuit and increased cost of the reproducing apparatus are highly undesirable in digital video signal reproducing apparatuses for home use where there is a demand for low cost.
On the other hand, the present inventors have devised a digital signal recording system disclosed in a Japanese Patent Application No. 56-139567. According to this devised system, one frame (or one field) of the video signal related to the color still picture information comprises the luminance signal and the two color difference signals (B-Y) and (R-Y), and three kinds of digital video signals are obtained by independently subjecting the luminance signal and the two color difference signals to digital pulse modulation. The three kinds of digital video signals are successively and time sequentially transmitted, and this devised system is in accordance with the component coding system.
However, according to this devised digital signal recording system, one frame (or one field) of the digital luminance signal is recorded, one frame (or one field) of the first digital color difference signal is recorded thereafter, and one frame (or one field) of the second digital color difference signal is recorded after the first digital color difference signal. Hence, if the operator attempts to change the still picture being displayed when reproducing a disc recorded according to such a recording system, the two kinds of color information will successively change in terms of one picture after the luminance information of the still picture of one picture changes. As a result, there was a disadvantage in that the reproduced picture was irregular and unpleasant to watch.
Accordingly, the present inventors have devised another recording system disclosed in Japanese Patent Applications No. 57-51925 through No. 57-51928. When recording the digital video signal onto the disc according to this devised system, the digital luminance signal and the two kinds of digital color difference signals are respectively transmitted alternately in terms of information corresponding to several lines or less. Moreover, a discrimination signal or header signal for enabling the reproducing apparatus to automatically discriminate the information content of the digital video signal and the like, is recorded at the beginning and end of the digital video signal corresponding to one frame or one field of the digital video signal transmitted with such a signal format. However, according to this devised system, if the digital video signal is reproduced from an intermediate point thereof due to an operation such as random access, it becomes impossible to read out from the memory circuit and the display of the read-out signal thus cannot be carried out for an interval until the subsequent digital video signal is reproduced. Furthermore, there was a problem in that it was impossible to partially modify the picture and carry out reproduction of a partially moving picture.